bo's'n vs old salt

bo's'n and old salt both are nouns.

Word	Noun	Adjective	Verb	Adverb
bo's'n	Yes	No	No	No
old salt	Yes	No	No	No

As nouns, old salt is a hypernym of bo's'n; that is, old salt is a word with a broader meaning than bo's'n:

bo's'n: a petty officer on a merchant ship who controls the work of other seamen
old salt: a man who serves as a sailor

Other hypernyms of bo's'n include gob, Jack-tar, Jack, mariner, sea dog, seafarer, seaman, tar.

bo's'n (noun)	old salt (noun)
a petty officer on a merchant ship who controls the work of other seamen	a man who serves as a sailor